An Interaction is the smallest piece of work that is carried out by a Contact Center Agent. Interactions are 'media agnostic' (or 'channel neutral') - all interactions share properties and actions that can be carried out on them regardless of their media type.
A Work Request groups all interactions carried out with a particular Customer. All work that has been carried out with a specific customer can then be reported on by the Contact Center - this information is used to build up a picture of the 'Customer Journey'.
Capabilities are set by the Server, and should be taken as an indication that the operation will succeed if attempted. Have a look at the Capabilities Tutorial to see them in action.